1. Field of the Invention
The present invention relates to a color image reading apparatus and, more particularly, to a color image reading apparatus having the function of performing a shading correction.
2. Related Background Art
Hitherto, a color film input apparatus using a line sensor such as a CCD or the like has been known as a color image reading apparatus which reads a color film and performs a certain signal conversion and, thereafter, obtains image information.
The inputting operation by the color film input apparatus will now be described with respect to the case of inputting a color negative film as a positive image as an example hereinbelow.
Generally, the color negative film has been subjected to a process called an orange mask and a negative image is recorded on an orange film. That is, the color negative film has an orange color in an unexposed portion and each pigment develops the color on the orange in an exposed portion.
In the conventional color film input apparatus, to remove the orange component from the color separation signals of three primary colors, namely, RGB (Red, Green, Blue) of a negative image as an original, a color negative film (hereinafter, referred to as a color base film or base film) which is not exposed and is the same kind as the original is prepared, the light transmitted through the base film is projected onto a sensor prior to reading the original film and a transmission concentration of the base film is read and stored into a memory such as an RAM or the like (hereinafter, the base film read signal is called shading correction data).
From the shading correction data, the orange component in the negative film is eliminated and a sensitivity variation of the CCD sensor and a light amount distribution variation of an original illuminating light source are also corrected.
There is also an apparatus in which after the shading correction data is read, the base film is replaced with an original film of an image to be read, the original film is projected by a light source and scanned and read by a CCD sensor, the image data and shading correction data are arithmetically process, and the read image of the original film is corrected (Japanese Laid-Open Patent Application No. 63-124665).
In the above conventional apparatus, however, each time the color negative film is read, in addition to the original film, a color base film of the same maker and the same kind as those of the original film must be prepared.
On the other hand, each time the color negative film is read, a troublesome operation to read the color base film is needed as a pre-process. Further, there is a drawback in that the film is bleached by the repetitive exposure or aging change of the color base film.